Comet ISON: Seven Things to Know About Comets

On Thursday, November 28, Comet ISON is slated to travel just 750,000 miles from the Sun—its perihelion, or closest approach to the Sun.

Discovered only in late 2012, comet ISON has been exciting from the start. Astronomers surmised immediately, “Oh, this could be big,” says Denton Ebel, chair of the Division of Physical Sciences, since “ISON was very bright and active even while it was in the outer solar system.” Dr. Ebel works on samples from NASA’s Stardust mission, which collected dust from Comet Wild2 and returned it to Earth.

Taken on Nov. 19, 2013, this image shows a composite "stacked" image of C/2012 S1 (ISON). These five stacked images of 10 seconds each were taken with the 20" Marshall Space Flight Center telescope in New Mexico. This technique allows the comet's sweeping tail to emerge with more detail.

NASA/MSFC/MEO/Cameron McCarty

Comet ISON wouldn't be visible from Earth for a time as it slingshot around the Sun, and observers are getting excited to have it come back into view from Earth in mid-December.

[Update: But that was not to be. "I have a note on my calendar for today—December 13—to 'view comet ISON,'" says Dr. Ebel. "But I guess that's not happening."

Why not? Many comets fragment as they encounter the intense heat and gravity of the Sun, and while at first skywatchers thought comet ISON might have survived the trip, it appears to have not. "It probably both fragmented and, basically, sublimed—that is, the ices became vaporized," says Dr. Ebel. Indeed, the comet appears to have broken up almost completely even before it reached its closest approach to the Sun, according to observers from NASA and elsewhere.]

Need a refresher course on comets? Here are seven things to know about these exciting but enigmatic Sun-orbiting space objects.

1. Like asteroids, comets are likely remnants of the formation of the solar system about 4.6 billion years ago.

2. Comets’ main components are rock, ices, and dust.

3. Comets orbit the Sun.

As they near the Sun, they develop a coma—a thin cloud of gas that forms directly from ice by “sublimation,” explains Ebel. They also develop two tails—one of dust, which can be millions of miles long, and one of ionized (electrically charges) gases. It’s the coma and the tails that can make comets such spectacular night-sky sightings.

4. While most asteroids orbit in the Asteroid Belt between Mars and Jupiter, comets come from farther away.

Short-period comets—those with orbits that last less than 200 years—originate in the Kuiper Belt. (Many dwarf planets, including Pluto, also reside in the Kuiper Belt.) Long-period comets, and some that will never return, like comet ISON, likely come from the more distant Oort cloud, which is thought to extend almost a light year from the Sun.

5. The Oort Cloud is hypothetical at this point.

Spacecraft have not traveled there; the Voyager spacecraft, for instance, have only gone to 16 light-hours from the Sun, and astronomers cannot observe it directly—but in 1950 astronomer Jan Oort hypothesized that such an area must exist. This isn’t as far-fetched as it may sound. In 1951, astronomer Gerard Kuiper hypothesized the Kuiper Belt, and it was only in 1992 that the first confirmed Kuiper Belt object was discovered.

6. Comets from the Oort Cloud only rarely enter the inner solar system.

“That’s fortunate,” says Ebel, because comets can be big (ISON is over 1 kilometer in size) and fast,” and potentially dangerous to Earth. As comets near Earth, they travel much faster than asteroids do, sometimes approaching 100 times the speed of sound on Earth—because they all obey Kepler’s laws of planetary motion.

7. There is still a lot to learn about comets.

“We know that comets are made of rock and ice,” explains Denton Ebel, “but we don’t know the proportions of each or their internal structure.” And that’s why it’s hard to predict how a comet will react as it nears the heat of the Sun. Some may be densely packed, while others may be, says Ebel, “like Swiss cheese!” Researchers are trying to learn more, however. After 2015, the unmanned New Horizons mission will enter the Kuiper Belt, in part to learn more about comets.